Simplify the following expression: $x = \dfrac{r}{5} - \dfrac{r}{9}$
Explanation: In order to subtract expressions, they must have a common denominator. The smallest common denominator is the least common multiple of $5$ and $9$ $\lcm(5, 9) = 45$ $ x = \dfrac{9}{9} \cdot \dfrac{r}{5} - \dfrac{5}{5} \cdot \dfrac{r}{9} $ $x = \dfrac{9r}{45} - \dfrac{5r}{45}$ $x = \dfrac{9r -5r}{45}$ $x = \dfrac{4r}{45}$